Stretched films obtained by stretching resin are used as optical films that achieve various optical functions in various types of display devices, utilizing their optical anisotropy. For example, it is known that in a liquid crystal display device, the stretched film is used as an optical compensation film such as for optical compensation such as coloring prevention and wide viewing angle, and that the stretched film is adhered to a polarizer, and thus the stretched film is used as a retardation film which also functions as a polarizing plate protective film.
On the other hand, in recent years, as a new type of display device, much attention has been focused on self-luminous display devices such as an organic EL (electroluminescence) display device. In the self-luminous display device, there is a room for reduction of power consumption as compared with a liquid crystal display device in which a backlight is constantly on. Furthermore, in the self-luminous display device, such as the organic EL display device, in which light sources corresponding to individual colors are individually turned on, since it is not necessary to provide a color filter that is a factor for reducing a contrast, it is possible to further increase the contrast.
However, since in the organic EL display device, in order to increase the efficiency of taking out light, a reflective member such as an aluminum plate is provided on the back surface side of a display, external light entering the display is reflected off the reflective member, with the result that an image contrast is disadvantageously reduced.
Hence, it is known that in order to enhance a light and dark contrast through external light reflection prevention, the stretched film is adhered to a polarizer to form a circular polarizing plate, and the circular polarizing plate is used on the surface side of the display. Here, the circular polarizing plate is formed by adhering the polarizer to the stretched film such that the in-plane slow axis of the stretched film is inclined at a desired angle with respect to the transmission axis of the polarizer.
However, a general polarizer (polarizing film) is obtained by being stretched at high magnification in a transport direction, and its transmission axis is aligned with a width direction. Thus, a conventional retardation film is manufactured by vertical stretching or lateral stretching and, in principle, the in-plane slow axis is pointed in a direction at 0° or 90° with respect to the longitudinal direction of the film. Hence, in order to incline, at a desired angle, the transmission axis of the polarizer and the slow axis of the stretched film as described above, there is no choice but to adopt a batch method of cutting a long polarizing film and/or a stretched film into film pieces at a particular angle and of adhering the film pieces one by one. This disadvantageously results in poor productivity and low product yield due to attachment of shavings or the like.
In order to overcome this problem, there are proposed various methods for manufacturing a long retardation film that allows a film to be stretched in a direction at a desired angle (in an oblique direction) with respect to the longitudinal direction and that allows the direction of the slow axis to be controlled to be an arbitrary direction neither at 0° nor at 90° with respect to the longitudinal direction of the film. For example, in the manufacturing method disclosed in patent document 1, a resin film is fed out from a direction different from the winding direction of the film after the stretching of the resin film, and is transported with both end portions of the resin film held with a pair of holding members. The transport direction of the resin film is changed partway and thus the resin film is stretched in an oblique direction. In this way, a long stretched film is manufactured that has a slow axis at a desired angle more than 0° but less than 90° with respect to the longitudinal direction.
By use of such a stretched film having the slow axis inclined with respect to the longitudinal direction, it is possible to manufacture a circular polarizing plate by adhering a long polarizing film and a stretched film on a roll to roll basis instead of adhering by a conventional batch method. Consequently, the productivity of the circular polarizing plate is dramatically enhanced, and its yield is significantly improved.
Incidentally, when the film is stretched while being transported, it is known that bowing is produced. The bowing refers to a behavior in which a straight line drawn in the width direction of the film before stretching becomes concave or convex toward the longitudinal direction of the film after stretching. When the bowing described above is produced, since the orientation axis of the film is displaced from a desired angle, it is impossible to obtain a film having satisfactory optical properties.
In this point, patent document 1 discloses that, in order to remove the bowing described above, it is preferable to widen the distance between rails on which a pair of holding members travel after the stretching of the film and to stretch again the film in the width direction.
However, when the film is stretched in an oblique direction with respect to the width direction of the film, the pair of holding members which hold both end portions of the film travel in the transport direction of the film with one moving relatively in a preceding manner and the other moving relatively in a delayed manner. Since as described above, the pair of holding members asymmetrically travel in the width direction of the film, as shown in FIG. 14, bowing B′ is asymmetrically produced in the width direction of a film F′. The asymmetrical bowing B′ in the width direction described above cannot be removed by re-stretching, in the width direction, the film F′ as disclosed in patent document 1, and consequently, it is impossible to reduce the displacement of an orientation angle from a desired angle over the width direction.